1. Field of the Invention
The present invention relates to magnetostrictive transducers or gauges. The magnetostrictive position transducer gauge described herein, incorporates a novel signal reception subsystem, which improves noise performance. This permits the construction of longer gauges, or gauges for use in noisier environments.
The inventive structure includes a threshold detector system which automatically compensates for the position of a location indicating magnet.
The invention is disclosed in the context of a "4-20 ma transmitter standard", magnetostrictive linear position transducer.
2. Description of the Prior Art
In general, magnetostrictive position sensors incorporate a ferromagnetic delay line, or "waveguide". A pulse generator supplies a current pulse to the delay line which generates a magnetic field which surrounds the delay line. A remote and movable, position indicating magnet is positioned along the delay line. The magnetic field of the position magnet disturbs the magnetic field generated by the current pulse.
The interaction between the permanent magnetic field of the position magnet and the magnetic field induced by the current pulse causes a strain or mechanical reaction within the delay line. This strain induced reaction force within the delay line, is propagated along the length of the delay line as a delayed acoustic torsional wave.
A device, called a mode converter, is typically attached to one end of the waveguide. This element responds to the passage of the torsional acoustic wave and converts it into a representative electrical signal.
The time delay period from the excitation of the waveguide to the reception of the corresponding acoustic wave at the mode converter indicates the position or location of the position magnet along the length of the delay line.
A variety of time measurement, or intervalometer techniques have been used to convert the time period information into a position indicating signal.
For example, U.S. Pat. No. 3,898,555 to J. Tellerman, uses a fixed frequency oscillator to excite the delay line. The returned acoustic signal, in conjunction with the fixed frequency oscillator, develops a signal which is "pulse width modulated" by the position of the magnet along the delay line. An integrator converts the pulse width modulated waveform to a dc voltage level which forms the transducer output.
U.S. Pat. No. 4,721,902 to J. Tellerman et al. teaches inter alia, a method to convert the "pulse width modulated signal" into a digital value. The patent teaches the use of a conversion counter to collect "counts" from a conversion oscillator during the "on" time of the pulse width modulated signal.
This patent also teaches a method to enhance the detection of the delayed acoustic signal through the use of a time domain filter which sets the duration of an inhibit timer based upon the historical output of the transducer. This time domain filtering technique eliminates the contribution of noise to the output signal, however it limits the rate at which the position indicating magnet can move along the gauge.
Magnetostrictive position sensor devices of this type are used in the measurement and control industry. They find use in machine tools; in robotics; as liquid level indicators, as well as other applications.
To facilitate the use of various types of transducers, produced by a variety of manufacturers, industry has adopted a current mode transducer standard referred to as the "4-20 milliamp transmitter" standard. Under this standard, transducers are supplied as a two terminal device. In use, the two terminal transducer device is coupled to a power supply (24 volts, D.C.) and the amount of current drawn by the transducer from the power supply indicates the measured value of the transduced signal. For example, a pressure sensor may draw 4 ma of current from the remote power supply at the minimum pressure, and 20 ma at the maximum pressure, while intermediate pressures would correspond to intermediate current draws.
The magnetostrictive measurement technique requires the reliable detection of the delayed acoustic pulse. These acoustic pulses are attenuated during the course of transmission in the waveguide. In general, the amplitude of the acoustic pulses are the greatest when position indicating magnet is closest to the mode converter; the acoustic pulses are faintest when the magnet is remote from the mode converter.
In the prior art, the maximum length of a magnetostrictive gauge was limited by the detectabilty, in the presence of noise, of the delayed acoustic pulse.